A conventional road surface comprises three courses, namely the bottom base course, the middle binder course and the top wearing layer, which differ in their composition and thus in their pore contents. Whereas the wearing course has a pore content of about 2.5% to 3% by volume, the pore content of the binder course lies within the range from about 5% to 7% by volume, the base course having a pore content within the range from about 10% to 12% by volume. The wearing course essentially provides a seal and should accordingly be as far as possible dense and impervious to water. The base course is essentially for supporting axle loads and the binder course is mainly for bearing tangential stress generated by moving traffic. The binder course commonly comprises a high proportion of chippings within the range of about 65% to 75% by weight. The composite material for the wearing course has the highest binder content, about 5.7% to 9% by weight of bitumen and the lowest proportion of chippings, about 30% to 60% by weight. "Chippings" may be defined as having a particle size greater than about 2 mms. By comparison, sand has a particle size of about 0.09 to 2 mms. Finally, the charge or filler, commonly a fine rock powder, has a particle size smaller than 0.09 mm.
Stabilising additives such as organic and inorganic fibrous substances, with or without admixtures of inorganic materials, native asphalt, diatomaceous earth, fossil meal, trass or volcanic tuff, artificial and natural silicic acids, synthetic resins and polymer-modified bitumen (PmB), may be added to prevent track furrows and thereby to improve the stability of asphalt coatings, as referred to in "Complementary Technical Regulations and Directives for the Construction of Bituminous Roadway Surfaces"-ZTV to StB 84.
These additives in bituminous composition material for the production of cast and rolled asphalt stabilize, that is to say improve the resistance to deformation of, cast and rolled asphalt, and thus improve the reinforcement of the mortar contents. Such additives, which may be described as reinforcing agents, bind bitumen by virtue of their very large specific surface area and thereby greatly increase the "internal friction" between particles of inorganic substance and bitumen films.
The additives primarily used are disadvantageously costly.
An object of the invention is, therefore, to provide a stabilising additive for composite bituminous material for the production of cast and rolled asphalt, which is more economical than the additives referred to above, but which is equally, or indeed more, effective.